1. Field of the Invention
The present invention relates to a straight traveling hydraulic circuit which can prevent a declination of equipment when traveling devices and working devices are simultaneously driven.
More particularly, the present invention relates to a straight traveling hydraulic circuit, which, in the case of performing a combined operation in which working devices and traveling devices are simultaneously driven, can prevent a declination of equipment due to an overload occurring in the working devices such a boom and so on.
2. Description of the Prior Art
As illustrated in FIG. 1, a conventional straight traveling hydraulic circuit includes first and second variable hydraulic pumps 15 and 18; a left traveling motor 2 and a first working device (not illustrated) such as an arm and so on, connected to the first hydraulic pump 15 and driven when hydraulic fluid is fed thereto; a plurality of switching valves 12 and 26 installed in a flow path 1 of the first hydraulic pump 15 and shifted to control the hydraulic fluid fed to the left traveling motor 2 and the first working device in response to pilot signal pressures a1 and b1 applied thereto; a right traveling motor 3 and a second working device (not illustrated) such as a boom and so on, connected to the second hydraulic pump 18 and driven when the hydraulic fluid is fed thereto; a plurality of switching valves 11 and 28 installed in a flow path 9 of the second hydraulic pump 18 and shifted to control the hydraulic fluid fed to the right traveling motor 3 and the second working device in response to pilot signal pressures a2 and b2 applied thereto; and a straight traveling valve 4 installed in the flow path 9 and shifted, in response to a pilot signal pressure a3 applied thereto, to supply the hydraulic fluid fed from the first hydraulic pump 15 to the left and right traveling motors 2 and 3, to supply a part of the hydraulic fluid fed from the second hydraulic pump 18 to a first working device switching valve 26 through a flow path 32, and simultaneously to supply a part of the hydraulic fluid fed from the second hydraulic pump 18 to a second working device switching valve 28 through a flow path 7.
In the drawing, reference numeral 10 denotes a main relief valve that protects a hydraulic system by making a part of the hydraulic fluid drain to a hydraulic tank 16 when an overload that exceeds a predetermined pressure in the hydraulic circuit occurs.
A) In the case of solely performing a traveling operation:
As the pilot signal pressure a1 is applied to the left traveling motor switching valve 12, an inner spool of the switching valve 12 is shifted in the left direction as shown in the drawing. Accordingly, the hydraulic fluid fed from the first hydraulic pump 15 is supplied to the left traveling motor 2 via the flow path 1, the switching valve 12, and a flow path 14.
As the pilot signal pressure a2 is applied to the right traveling motor switching valve 11, an inner spool of the switching valve 11 is shifted in the right direction as shown in the drawing. Accordingly, the hydraulic fluid fed from the second hydraulic pump 18 is supplied to the right traveling motor 3 via the flow path 9, the switching valve 11, and a flow path 20.
That is, in the case of solely driving the left traveling motor 2 or the right traveling motor 3, the hydraulic fluid fed from the first hydraulic pump 15 is supplied to the left traveling motor 2, and the hydraulic fluid fed from the second hydraulic pump 18 is supplied to the right traveling motor 3.
B) In the case of performing a combined operation by simultaneously driving the traveling devices and the working devices:
As the pilot signal pressure a3 is applied to a straight traveling valve 4, an inner spool of the straight traveling valve 4 is shifted in the right direction as shown in the drawing. Simultaneously, as the pilot signal pressure b1 is applied to the first working device switching valve 26, an inner spool of the switching valve 26 is shifted in the left direction as shown in the drawing. Accordingly, a signal pressure c1 is applied to a first center bypass valve 22 to shift an inner spool of the bypass valve 22 in the left direction, and thus a pressure is formed in a first center bypass flow path.
Accordingly, a part of the hydraulic fluid fed from the first hydraulic pump 15 is supplied to the left traveling motor 2 via the flow path 1, the switching valve 12, and the flow path 14. Simultaneously, a part of the hydraulic fluid fed from the first hydraulic pump 15 is supplied to the right traveling motor 3 via a flow path 8, the straight traveling valve 4, the switching valve 11, and the flow path 20.
That is, the hydraulic fluid fed from the first hydraulic pump 15 is used to drive the left traveling motor 2 and the right traveling motor 3.
On the other hand, the hydraulic fluid fed from the second hydraulic pump 18 is supplied to the first working device switching valve 26 via the flow path 9, the straight traveling valve 4, and the flow path 32, to drive the corresponding working devices such as the arm and so on.
That is, the hydraulic fluid fed from the second hydraulic pump 18 is supplied to the first working device switching valve 26 and is used to drive the corresponding devices.
On the above-described condition, if the pressure for shifting the first working device switching valve 26 is gradually increased and its inner spool is shifted to a full-stroke state, the pressure is increased up to a predetermined pressure of the main relief valve 10. In this case, the hydraulic fluid fed from the second hydraulic pump 18 is not supplied to the first working device switching valve 26 any more.
That is, a part of the hydraulic fluid being supplied to the switching valve 26 passes through the flow path 32, the straight traveling valve 4, the flow path 9, and the flow path 7, and then is supplied to the right traveling motor 3 via a check valve 5 an orifice 6. A part of the hydraulic fluid being supplied to the switching valve 26 is supplied to the left traveling motor 2 via the flow path 8.
In this case, the traveling motor switching valves 12 and 11 are shifted in response to the pilot signal pressures a1 and a2 being applied thereto. During the combined operation, the traveling side pilot signal pressure, which is kept about 10˜12K, shifts the switching valves 12 and 11. Accordingly, in the case where the traveling motor switching valves 11 and 12 are half shifted, it is possible to control the flow of the hydraulic fluid by a P-N notch (that controls the hydraulic fluid fed from the hydraulic pump to the hydraulic tank), a P-C notch (that controls the hydraulic fluid fed from the hydraulic pump to a hydraulic cylinder), and a C-T notch (that controls the hydraulic fluid fed from the hydraulic cylinder to the hydraulic tank).
In the conventional hydraulic circuit, no hydraulic fluid is fed through the P-N notch when the switching valve 26 and the first center bypass valve 22 are shifted. Accordingly, the hydraulic fluid fed through the switching valves 11 and 12 can be controlled by the P-C notch or the C-T notch.
The spool notches of the traveling motor switching valves 11 and 12 have the same structure. However, due to the difference in accumulated tolerance and processing condition between the spools, it is difficult for the spool notches to keep the same cross-sectional area.
That is, since the flow rate of the hydraulic fluid passing through the spool is in proportion to the cross-sectional area of the spool, the flow rate of the hydraulic fluid passing through the traveling motor switching valve 12 is different from that passing through the traveling motor switching valve 11 when the cross-sectional areas of the spool notches are different from each other. In the case where the flow rates of the traveling motor switching valves 12 and 11 are different from each other, the driving speed of the traveling motor on the side where the flow rate is relatively high becomes abruptly high, whereas the driving speed of the traveling motor on the side where the flow rate is low becomes lowered.
As described above, in the case of performing the combined operation in which the traveling motors 2 and 3 are driven in a state that the spools of the traveling motor switching valves 12 and 11 are half shifted (at this time, the spool of the straight traveling valve 4 is fully shifted), and the working devices such as the boom and so on are driven simultaneously with the traveling devices, a declination of the equipment may occur due to an overload occurring in the working devices.